Integrated circuits such as microprocessors generate heat when they operate and frequently this heat must be dissipated or removed from the integrated circuit die to prevent overheating. This is particularly true when the microprocessor is used in a notebook computer or other compact device where space is tightly constrained and more traditional die cooling techniques such as direct forced air cooling are impractical to implement.
One technique for cooling an integrated circuit die is to attach a fluid-filled microchannel heat exchanger to the die. A typical microchannel heat exchanger consists of a silicon substrate in which microchannels have been formed using a subtractive microfabrication process such as deep reactive ion etching or electro-discharge machining. Typical microchannels are rectangular in cross-section with widths of about 100 m and depths of between 100–300 m. Fundamentally the microchannels improve a heat exchanger s coefficient of heat transfer by increasing the conductive surface area in the heat exchanger. Heat conducted into the fluid filling the channels can be removed simply by withdrawing the heated fluid.
Typically, the microchannel heat exchanger is part of a closed loop cooling system that uses a pump to cycle a fluid such as water between the microchannel heat exchanger where the fluid absorbs heat from a microprocessor or other integrated circuit die and a remote heat sink where the fluid is cooled. Heat transfer between the microchannel walls and the fluid is greatly improved if sufficient heat is conducted into the fluid to cause it to vaporize. Such two-phase cooling enhances the efficiency of the microchannel heat exchanger because significant thermal energy above and beyond that which can be simply conducted into the fluid is consumed in overcoming the fluid s latent heat of vaporization. This latent heat is then expelled from the system when the fluid vapor condenses back to liquid form in the remote heat sink. Water is a particularly useful fluid to use in two-phase systems because it is cheap, has a high heat (or enthalpy) of vaporization and boils at a temperature that is well suited to cooling integrated circuits.
The heat removal capacity of microchannel heat exchangers can be enhanced by vertically stacking multiple layers of microchannel structures to form a stacked microchannel heat exchanger. Stacked microchannel heat exchangers are more efficient at removing heat from ICs because each additional layer of microchannels doubles the surface area for heat exchange per unit area of the heat exchanger.
Conventionally, heat exchangers are not physically coupled directly to an IC die or package but, rather, are coupled to a metallic heat spreader that is itself coupled to the IC die or package. In the context of mobile computing systems the size of a typical heat exchanger often precludes coupling the heat exchanger directly to the heat spreader thus requiring the addition of a heat pipe or other thermally conductive structure to provide the physical and thermal coupling between the heat exchanger and the heat spreader. Heat pipes or similar devices are bulky and occupy valuable space within a mobile computing system.